The present invention relates to automatic frequency control methods in digital receivers and more particularly, to an automatic frequency control method applicable to even a digital receiver using a receiving signal of modulation type such as phase shift keying (PSK) or quadratic amplitude modulation (QAM).
In the receiver, an internal oscillator has an error in frequency and therefore, a detected signal suffers from a frequency error because of an error between a carrier wave frequency of a received signal and a frequency to be demodulated. Especially, in the digital receiver using the receiving signal of the PSK or QAM modulation type, a frequency error causes an error in demodulated data and hence, there is a need for automatic frequency control adapted to correct the frequency error.
Techniques for controlling frequency errors in radio communication apparatus are disclosed in JP-A-2000-324080, JP-A-9-246916, JP-A-11-308157 and JP-A-7-297779. Any of the conventional frequency error control techniques in radio communication apparatus could not afford to perform a frequency error correction operation capable of rapidly and accurately converging the frequency to a correct frequency even in the event that frequency errors in a wide range take place in the receiving and transmitting apparatus.